The present invention generally relates to control circuits and more particularly to an on-off control circuit between an IEEE 1394a and IEEE 1394b compliant physical layer (PHY) output driver circuitry and a glass fiber optical physical medium dependent (PMD) sub-layer within the architecture of the IEEE 1394b standard.
Based upon technology developed by Apple Computer, Inc. of Cupertino, Calif., FireWire (IEEE 1394a or IEEE 1394-1995) was adopted in 1995 as an industry standard for cross-platform peripheral connectivity. The IEEE1394 serial bus is an easy, cost-effective, and high-performance means of interconnecting high-speed storage, imaging, and many other types of audio/video (ANV) devices, providing a convenient network interface, and being capable of supporting both asynchronous data transport such as IP as well as isochronous media streaming (AV/C).
In the IEEE 1394a standard, the device addressing is 64 bits wide, partitioned as 10 bits for bus ID, 6 bits for node ID and 48 bits for memory addresses. The result is the capability to address up to 1023 buses, with each having up to 63 nodes. Each node acts as a repeater, allowing nodes to be chained together to form a tree topology.
An updated version of the IEEE 1394a standard, IEEE 1394b, was released in 2002. The updated standard doubles the throughput of the original technology, dramatically increases the maximum distance of FireWire connections, and supports many new types of cabling. IEEE 1394b allows extensions to 800 Mbit/sec., and 1.6 Gbit/sec. and 3.2 Gbit/sec. over copper wires. The updated standard supports long-distance transfers to 100 meters over a variety of media: CAT-5 unshielded cable at 100 Mbit/sec., existing plastic optical fiber at 200 Mbits/sec., next-generation plastic optical fiber at 400 Mbit/sec. and 50-micron multimode glass optical fiber at up to 3.2 Gbit/sec. The improved speed and distance capabilities of IEEE 1394b result from two major improvements; overlapped arbitration and advanced 8B10B encoding scrambling both data and control symbols used by Fiber Channel and Gigabit Ethernet. This scrambling code results in much lower average attenuation—more than 20 dB below unscrambled codes. Together, the two improvements are called the beta mode of operation to distinguish it from the IEEE 1394a legacy mode.
Because the IEEE 1394b standard is backward compatible with the original IEEE1394a standard, the IEEE 1394b standard defines an IEEE 1394 PHY to be bilingual capable, that is, the IEEE 1394 PHY is capable of working either with a peer IEEE1394 device (beta mode) or with an IEEE1394a device (legacy mode). A mode determination algorithm is embodied within the connection management protocol at the very beginning of a connection establishing process.
In order to establish a connection between optical ports of two peer nodes interconnected with a glass fiber optical cable, a toning signal is generated and transmitted on Twisted Pair B (TPB+/−) of the IEEE 1394b PHY. Toning is the IEEE 1394b method for beta-mode connection determination. Toning makes use of a “tone” signal and a “tone” detector. An IEEE 1394b device is also listening to its peer port from a Twisted Pair A (TPA+/−) of its own, acting as a “tone” detector. The node is ready to detect the incoming signal as a tone if the peer device has an IEEE 1394b PHY or as a TPBias if the peer device has an IEEE 1394a PHY. When the peer node receives a tone signal, it recognizes that the other node is an IEEE 1394b device and a beta mode connection may be established.
The toning signal has a frequency of 50 MHz with a duration of 667 μsec and a period of 42.7 msec. At the receiver of an IEEE 1394 PHY, the tone must be a minimum of 400 μsec wide to be considered a valid tone. In other words, an IEEE 1394b PHY needs to see a toning signal of at least 400 μsec duration but not longer than 667 μsec duration. This gives the system a tolerance margin of 227 μsec as a timing budget to handle transmission loss considering the propagation delay caused by other in-line devices such as the on-off controller and the optical transceivers. During the tone the transmitter TPB signals are compliant to the level as specified in the IEEE 1394b standard. Between tones the transmitter is placed in high impedance or it is in a so-called quiet period when it is in the connection detect phase.
Once a valid tone is detected between the two connected IEEE 1394b nodes, a speed negotiation sequence is entered. Speed negotiation is a process in which two connected IEEE 1394b nodes can establish continuous operation at the correct operating speed and avoid any need for matching configuration options at “both” ends to determine what a “common denominator” operating speed should be. The same toning signals that were used in connection detection are again used in speed negotiation but the tones are sent in different patterns. Tones of the same electrical characteristics are sent at specified intervals to indicate speed capabilities. Both ports transmit a speed code indicating their respective maximum speeds, while simultaneously listening for the speed codes from the peer port. The ports then transmit the minimum of the transmitted and received speeds, together with an acknowledge bit (to indicate that the agreed speed has been received). When a port receives a speed code with the acknowledge bit set, then it ceases to transmit speed codes, and the connection is established. The speed code timing and encoding for this purpose is shown in FIG. 1.
Following the speed negotiation process, the nodes then can start sending symbols and align their clocks (also called training). Once the training sequence is completed, idle symbols are continually sent until data transfers are started.
The IEEE 1394b PHY receiver can only accept a “1”, “0” or “z”(high impedance). The “z” means an open circuit or the input is floated. It is strictly constrained that between each tone nothing may be transmitted from the peer optical transmitter. The receiver must see a “z” from its input circuit. Noise transmitted from the peer optical transmitter may confuse the IEEE 1394b PHY receiver and disturb the initialization process. Noise may prevent the establishment of a connection between the two IEEE 1394b nodes.
A prior art solution to the problem of noise known as biasing-off is shown in FIG. 2. The biasing-off solution achieves noise elimination by unbalancing the bias voltage of the differential pair. This method can only be used in a system that has a DC coupled optical transceiver. With an AC coupled optical transceiver it is impossible to change the bias level from outside of the transceiver itself. As shown in FIG. 2, the LVPECL is normally biased at a level of 2.1V. By slightly lifting the bias of the differential pair's negative pole to 2.4V, the low 300 MV range is squeezed out. Because normal noise falls into this range the noise is “biased off” by the 300 MV bias unbalance between the differential pair. Although the approach works well in some cases, its disadvantages are obvious. Firstly, it can only work with a DC coupled optical transceiver. It will not work with systems having AC coupled optical transceivers. Secondly, because systems using such a solution apply a non-standard unbalanced bias, not only is the noise eliminated but the signal quality suffers from the unbalanced bias voltage at the same time. For these reasons this solution is not recommended by most component suppliers.
Another prior art solution to the problem of noise provides for the use of an optical transceiver having a squelched receiver. This solution suffers the disadvantage of being unreliable and costly.
There is therefore a need in the art for a control circuit between an IEEE 1394a and IEEE 1394b compliant physical layer (PHY) output driver circuitry and a glass fiber optical physical medium dependent (PMD) sub-layer within the architecture of the IEEE 1394b standard that turns off the optical transmitter during the periods between two consecutive tones and turns on the optical transmitter when a tone is present. Preferably the control circuit provides for smooth connection detection.